In some oil field applications it is desirable to employ a fluid powered reversible stepping actuator for setting a valve position. Such an actuator can be quite useful where the valve is in a remote location. For example, such a valve and actuator may be located at a sea floor wellhead with control provided from a platform or on-shore location.
A stepping actuator is desirable in many such applications because of the precise control easily obtainable. Such a stepping actuator may be used, for example, for rotating a valve stem just a few degrees for each step. The rotation of the stepping actuator may be only three degrees per pulse of pressure and if desired this can be geared down to rotate a valve stem. Thus, it is easy and straightforward to provide precise control of valve position.
An exemplary fluid powered reversible stepping actuator is described in U.S. Pat. No. 4,403,523. In such an actuator each pulse of pressure applied to the actuator rotates the valve stem through a known angle. So long as the pulse duration is above some minimum, the stepping actuator will move one increment and only one increment for each pulse, even though the pulse may be of long duration. Thus, excessive rotation of the stepping actuator is easily avoided.
It is desirable to provide a control system for such a reversible stepping actuator which provides pulses of pneumatic pressure with the pulse length and time interval between pulses being independently controllable. This assures that the actuator will execute each step regardless of delays in the lines between the control system and actuator. The control system should provide pulses that will drive the actuator in either direction as required to reach a desired setting.
It is often desirable that such a control system be entirely pneumatic. This can avoid use of electrical sensors and control devices where such usage is inconvenient or hazardous. It can often be desirable to have such a system operate only when a change in valve setting is desired, since in many applications a valve position may be set and remain unchanged for days or weeks. It is desirable in such a situation to have the control system stand by without power consumption.
It is desirable that such a system operate automatically in response to a control setting. That is, one should be able to provide a control signal indicative of a desired valve position and have the system automatically operate the stepping actuator until the desired position is reached. This eliminates need for manual adjustment of the valve and provides the option of either manually or automatically setting the desired valve position.